Driving method for discharge lamp, driving device for discharge lamp, light source device, and image display apparatus

ABSTRACT

In at least one embodiment of the disclosure, a driving device for a discharge lamp includes an alternating current supply section and a frequency modulation section. The alternating current supply section supplies two electrodes of the discharge lamp with an alternating current. The alternating current comprises a plurality of modulation periods. The frequency modulation section modulates a frequency of the alternating current so as to provide a plurality of retentive periods within each of the modulation periods. Each retentive period has a constant frequency that is different from a frequency of its temporally adjacent retentive periods. The frequency modulation section shortens a length of at least one of the retentive periods in the modulation period in response to a predetermined condition occurring. The frequency of at least one of the retentive periods is equal to or less than a predetermined reference frequency.

CROSS-REFERENCE

The present application claims priority from Japanese Patent ApplicationNo. 2008-310805 filed on Dec. 5, 2008 and Japanese Patent ApplicationNo. 2009-134872 filed on Jun. 4, 2009, each of which is herebyincorporated by reference in its entirety.

BACKGROUND

Discharge lamps such as super-high pressure discharge lamps may be usedas a light source in image display apparatuses such as projectors. Insuch a super high-pressure discharge lamp, the arc used as a luminescentspot is formed between projections provided to the electrodes.Therefore, it has been proposed that the projections are formed on thetips of the electrodes, and in order for forming the arc originated fromthe projections, an alternating current having a signal with a frequencylower than a stationary frequency inserted in a signal with thestationary frequency is supplied to the super-high pressure dischargelamp (see, e.g., Japan Patent Publication No. JP-A-2006-59790).

However, even if an alternating current having a signal with a lowerfrequency inserted is supplied to the discharge lamp, projections withpreferable shapes may not be formed depending on the conditions of theelectrodes, and flicker caused by migration of the bright spot of thearc might occur.

SUMMARY

Various embodiments of the disclosure make it possible to more reliablyprevent the flickers from occurring.

In certain embodiments, there is provided a driving device for adischarge lamp including an alternating current supply section adaptedto supply an alternating current between two electrodes of the dischargelamp, and a frequency modulation section adapted to modulate a frequencyof the alternating current by providing a plurality of periods withfrequencies of the alternating current different from each other withina period of modulation, and the frequency modulation section makes atleast one period of the plurality of periods with the frequency one ofequal to and lower than a predetermined reference frequency shorter thanbefore a predetermined condition is satisfied in response to thepredetermined condition being satisfied.

When keeping the state in which the frequency of the alternating currentis set to be low, the possibility of occurrence of the flicker becomeshigh. Since in this aspect, the time period during which the dischargelamp is driven at low frequency can be shortened by shortening at leastone period with the frequency equal to or lower than the referencefrequency, it becomes possible to prevent the flicker from occurring.

According to another aspect, the frequency modulation section, inperforming the shortening, shortens a length of a first period includedin the plurality of periods and having the frequency corresponding to afirst frequency to be shorter than a length of a second period includedin the plurality of periods and having the frequency corresponding to asecond frequency higher than the first frequency.

In general, when the frequency of the alternating current is set to behigh, the tip of the electrode extends toward the electrode opposedthereto, and thus the electrode is deformed to have a shape suitable forpreventing the flicker. According to this aspect, since the secondperiod with the higher frequency can be made longer than the firstperiod with the lower frequency, it becomes possible to deform theelectrode to have the shape suitable for preventing the flicker.

According to another aspect, the frequency modulation section, inperforming the shortening, shortens the period in accordance with thefrequency of the period so that the lower the frequency is, the shorterthe period becomes.

According to this aspect, since the period with the higher frequency canbe set to be longer, it becomes easier to deform the electrode to have ashape suitable for preventing the flicker.

According to another aspect, the frequency modulation section shortenseach of the plurality of periods in response to the predeterminedcondition being satisfied.

According also to this aspect, since the period with the frequency equalto or lower than the reference frequency can be shortened, it becomespossible to prevent the flicker from occurring.

According to another aspect, there is further provided a flickerdetection section adapted to detect occurrence of a flicker in thedischarge lamp as a criterion of the predetermined condition, and thefrequency modulation section performs the shortening upon detection ofthe flicker in the period with the frequency equal to or lower than thereference frequency.

According to this aspect, since the period with the frequency at whichthe flicker occurs actually can be shortened, it becomes possible tomore reliably prevent the flicker.

According to another aspect, the frequency modulation sectiondetermines, after predetermined time has elapsed since the shortening,whether or not the flicker occurs at the frequency at which the flickerhas been detected, and restores the period shortened once to have alength the shortened period had before shortening upon occurrence of noflicker.

If the shortened length of the period is restored before sufficientperiod of time has elapsed since the shortening, modification of theshape of the electrode is not sufficient, and there is a highpossibility of reoccurrence of the flicker. Therefore, since it ispossible to sufficiently modify the shape of the electrode bydetermining whether or not the restoration should be performed after thepredetermined time has elapsed, it becomes possible to prevent thereoccurrence of the flicker.

According to another aspect, the predetermined condition is determinedbased on a deterioration state of the discharge lamp, and the frequencymodulation section performs the shortening in response to determinationthat deterioration of the discharge lamp is in progress.

In general, when the deterioration of the discharge lamp is advanced,the electrodes are consumed, which makes the flicker be apt to occur.According to this aspect, in the case in which the deterioration of thedischarge lamp, which thus makes the flicker be apt to occur, isadvanced, by shortening at least one period with the frequency equal toor lower than the reference frequency, the flicker can more reliably beprevented from occurring.

According to another aspect, there is further provided with a lightingtime accumulation section adapted to calculate accumulated lighting timefrom beginning of use of the discharge lamp as a parameter representingthe deterioration state, and the frequency modulation section performsthe shortening in response to the accumulated lighting time exceeding apredetermined upper time limit.

According to this aspect, it becomes possible to more easily determinethe deterioration state of the discharge lamp.

According to another aspect, the alternating current supply section isconfigured to be able to modify power of the alternating current to besupplied to the discharge lamp, and the frequency modulation sectionperforms the shortening in response to the power of the alternatingcurrent to be supplied to the discharge lamp being lower thanpredetermined reference power.

In general, since when the discharge lamp is driven with low power, thetemperature of the electrode decreases, the possibility of occurrence ofthe flicker rises due to the continuous low frequency driving. Accordingto this aspect, since the time period during which the discharge lamp isdriven with the low frequencies can be made shorter in the case in whichthe power of the alternating current supplied to the discharge lamp islower than the predetermined reference power, even in the case in whichthe discharge lamp is driven with the low power, the flicker can beprevented from occurring.

According to another aspect, there is provided a driving device for adischarge lamp including an alternating current supply section adaptedto supply an alternating current between two electrodes of the dischargelamp, and a frequency modulation section adapted to modulate a frequencyof the alternating current by providing a plurality of periods withfrequencies of the alternating current different from each other withina period of modulation, and the frequency modulation section makes atleast one period of the plurality of periods with the frequencyexceeding a predetermined reference frequency longer than before apredetermined condition is satisfied in response to the predeterminedcondition being satisfied.

In the case in which the frequency of the alternating current is high,there is a tendency that a minute projection with a low thermal capacityis provided at the tip portion of the electrode. Further, in the case inwhich the fusibility of the electrode becomes deteriorated, theprojection with a planarized tip is provided at the tip portion of theelectrode when supplying the alternating current with a low frequency,and such a projection is not sufficiently melted because of a highthermal capacity thereof. As such, unevenness may be formed on thesurface of the projection and the possibility of the occurrence offlicker rises. According to this aspect, when the predeterminedcondition is satisfied, the minute projection is provided at the tipportion of the electrode to reduce the thermal capacity of theprojection by elongating at least one period with the frequencyexceeding the reference frequency, thus the fusibility of the projectioncan be enhanced. Thus, it is possible to prevent the occurrence of theflicker.

It should be noted that the disclosure can be put into practice invarious forms. The disclosure may be put into practice in the forms of,for example, a driving device and a driving method for a discharge lamp,a light source device using a discharge lamp and a control methodthereof, and an image display apparatus using the light source device.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present disclosurewill now be described with reference to the accompanying drawings,wherein like numbers reference like elements.

FIG. 1 is a schematic configuration diagram of a projector.

FIG. 2 is an explanatory diagram illustrating a configuration of a lightsource device.

FIG. 3 is a block diagram illustrating a configuration of a dischargelamp driving device.

FIGS. 4A and 4B are explanatory diagrams illustrating an example of amodulation pattern of a drive frequency.

FIGS. 5A, 5B, and 5C are explanatory diagrams illustrating how a shapeof a primary mirror side electrode varies when driving the dischargelamp at different drive frequencies.

FIGS. 6A and 6B are explanatory diagrams illustrating an influenceexerted by passage of time on a generation state of the arc in a lowfrequency driving mode.

FIG. 7 is a diagram illustrating how the modulation pattern is modifiedin a first embodiment.

FIG. 8 is a diagram illustrating how the modulation pattern is modifiedin a second embodiment.

FIG. 9 is a block diagram illustrating a configuration of a dischargelamp driving device in a third embodiment.

FIG. 10 is a flowchart illustrating a flow of a process for modifyingthe modulation pattern in the third embodiment.

FIG. 11 is a diagram illustrating how the modulation pattern is modifiedin a third embodiment.

FIG. 12 is a diagram illustrating how the modulation pattern is modifiedin a fourth embodiment.

FIG. 13 is a diagram illustrating how the modulation pattern is modifiedin a fifth embodiment.

FIGS. 14A and 14B are explanatory diagrams illustrating how the arcoccurs when the low frequency drive is continued in a low power drivingmode.

FIGS. 15A and 15B are explanatory diagrams illustrating a modifiedexample of the electrode provided to the discharge lamp in comparisonwith the embodiments described above.

DESCRIPTION OF EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and in which are shown, by way ofillustration, specific embodiments in which the disclosure may bepracticed. It is to be understood that other embodiments may be utilizedand changes may be made without departing from the scope of the presentdisclosure. Therefore, the following detailed description is not to betaken in a limiting sense, and the scope of the present disclosure isdefined by the appended claims and their equivalents.

Throughout the specification and claims, the following terms take atleast the meanings explicitly associated herein, unless the contextclearly dictates otherwise. The meanings identified below are notintended to limit the terms, but merely provide illustrative examplesfor use of the terms. The meaning of “a,” “an,” “one,” and “the” mayinclude reference to both the singular and the plural. Reference in thespecification to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment may be included in at least one embodiment of thedisclosure. The appearances of the phrases “in one embodiment” or “in anembodiment” in various places in the specification do not necessarilyall refer to the same embodiment, but it may. Several embodiments willsequentially be described under corresponding section headings below.Section headings are merely employed to improve readability, and theyare not to be construed to restrict or narrow the present disclosure.For example, the order of description headings should not necessarily beconstrued so as to imply that these operations are necessarily orderdependent or to imply the relative importance of an embodiment.Moreover, the scope of a disclosure under one section heading should notbe construed to restrict or to limit the disclosure to that particularembodiment, rather the disclosure should indicate that a particularfeature, structure, or characteristic described in connection with asection heading is included in at least one embodiment of thedisclosure, but it may also be used in connection with otherembodiments.

A1. Configuration of Projector

FIG. 1 is a schematic configuration diagram of a projector 1000 to whicha first embodiment is applied. The projector 1000 includes a lightsource device 100, an illumination optical system 310, a colorseparation optical system 320, three liquid crystal light valves 330R,330G, 330B, a cross dichroic prism 340, and a projection optical system350.

The light source device 100 has a light source unit 110 attached with adischarge lamp 500, and a discharge lamp driving device 200 for drivingthe discharge lamp 500. The discharge lamp 500 is supplied with electricpower by the discharge lamp driving device 200 to emit light. The lightsource unit 110 emits the light, which is emitted from the dischargelamp 500, toward the illumination optical system 310. It should be notedthat specific configurations and specific functions of the light sourceunit 110 and the discharge lamp driving device 200 will be describedlater.

The illumination optical system 310 uniformizes the illuminance of thelight emitted from the light source unit 110, and at the same timealigns the polarization direction thereof to one direction. The lightwith the illuminance uniformized through the illumination optical system310 and with the polarization direction aligned therethrough isseparated into three colored light beams of red (R), green (G), and blue(B) by the color separation optical system 320. The three colored lightbeams thus separated by the color separation optical system 320 aremodulated by the respective liquid crystal light valves 330R, 330G,330B. The three colored light beams respectively modulated by the liquidcrystal light valves 330R, 330G, 330B are then combined by the crossdichroic prism 340, and enter the projection optical system 350. By theprojection optical system 350 projecting the light beams, which haveentered, on a screen not shown, an image is displayed on the screen as afull color picture obtained by combining the images respectivelymodulated by the liquid crystal light valves 330R, 330G, 330B. It shouldbe noted that although in the first embodiment, the three liquid crystallight valves 330R, 330G, 330B individually modulate the respective threecolored light beams, it is also possible to assume that a single liquidcrystal light valve provided with a color filter modulates the lightbeam. In this case, it becomes possible to eliminate the colorseparation optical system 320 and the cross dichroic prism 340.

A2. Configuration of Light Source Device

FIG. 2 is an explanatory diagram showing a configuration of the lightsource device 100. As described above, the light source device 100 isprovided with the light source unit 110 and the discharge lamp drivingdevice 200. The light source unit 110 is provided with the dischargelamp 500, a primary reflecting mirror 112 having a spheroidal reflectingsurface, and a collimating lens 114 for obtaining an approximatelycollimated light beam as the light beam emitted therefrom. It should benoted that the reflecting surface of the primary reflecting mirror 112is not necessarily required to have a spheroidal shape. For example, thereflecting surface of the primary reflecting mirror 112 can have aparaboloidal shape. In this case, by placing the light emitting sectionof the discharge lamp 500 at a so-called focal point of the paraboloidalmirror, the collimating lens 114 can be eliminated. The primaryreflecting mirror 112 and the discharge lamp 500 are bonded to eachother with an inorganic adhesive 116.

The discharge lamp 500 is formed by bonding a discharge lamp main body510 and a secondary reflecting mirror 520 having a spherical reflectingsurface to each other with an inorganic adhesive 522. The discharge lampmain body 510 is formed from, for example, a glass material such asquartz glass. The discharge lamp main body 510 is provided with twoelectrodes 532, 542 formed from a high-melting point electrode materialsuch as tungsten, two connection members 534, 544, and two electrodeterminals 536, 546. The electrodes 532, 542 are disposed so that the tipportions thereof are opposed to each other in a discharge space 512formed at the central part of the discharge lamp main body 510. In thedischarge space 512, there is encapsulated a gas including a noble gas,mercury or a metallic halide, and so on as a discharge medium. Theconnection members 534, 544 are members for electrically connecting theelectrodes 532, 542 and the electrode terminals 536, 546 to each other,respectively.

The electrode terminals 536, 546 of the discharge lamp 500 areseparately connected to the discharge lamp driving device 200. Thedischarge lamp driving device 200 supplies the electrode terminals 536,546 with a pulsed alternating current (an alternating pulse current).When the alternating pulse current is supplied to the electrodeterminals 536, 546, an arc AR occurs between the tip portions of the twoelectrodes 532, 542 disposed in the discharge space 512. The arc ARemits light in all directions from the point where the arc AR occurs.The secondary reflecting mirror 520 reflects the light, which is emittedtoward the electrode 542, namely one of the electrodes 532, 542, towardthe primary reflecting mirror 112. By thus reflecting the light, whichis emitted toward the electrode 542, toward the primary reflectingmirror 112, a higher degree of parallelization of the light emitted fromthe light source unit 110 can be obtained. It should be noted that theelectrode 542 on the side where the secondary reflecting mirror 520 isdisposed is referred to also as a “secondary mirror side electrode 542,”and the other electrode 532 is referred to also as a “primary mirrorside electrode 532.”

FIG. 3 is a block diagram showing a configuration of the discharge lampdriving device 200. The discharge lamp driving device 200 has a drivecontrol section 210 and a lighting circuit 220. The drive controlsection 210 is configured as a computer provided with a CPU 610, a ROM620, a RAM 630, a timer 640, an output port 650 for outputting a controlsignal to the lighting circuit 220, and an input port 660 for acquiringa signal from the lighting circuit 220. The CPU 610 of the drive controlsection 210 executes a program stored in the ROM 620 based on outputsignals from the timer 640 and the input port 660. Thus, the CPU 610realizes functions as a drive frequency setting section 612, amodulation pattern setting section 614, and a lighting time accumulationsection 616.

The lighting circuit 220 has an inverter 222 for generating analternating pulse current. The lighting circuit 220 controls theinverter 222 based on the control signal supplied from the drive controlsection 210 via the output port 650. Specifically, the lighting circuit220 makes the inverter 222 generate the alternating pulse currentcorresponding to feed conditions (e.g., a frequency and a pulse waveformof the alternating pulse current) designated by the control signal. Theinverter 222 generates the alternating pulse current with constant power(e.g., 200 W) to be supplied to the discharge lamp 500 in accordancewith the feed conditions designated by the lighting circuit 220, andsupplies the discharge lamp 500 with the alternating pulse current thusgenerated.

The lighting time accumulation section 616 accumulates the lighting time(accumulated lighting time) of the discharge lamp 500 from the beginningof the use of the discharge lamp 500. Specifically, when the power isapplied to the discharge lamp driving device 200, the accumulatedlighting time at the time when the power has been cut previously isobtained from a rewritable area of the ROM 620 or a nonvolatile area ofthe RAM 630. Then, the accumulated lighting time is calculated based onan interval signal supplied from the timer 640 during the period inwhich the discharge lamp 500 is lighted, and the accumulated lightingtime thus calculated when the power of the discharge lamp driving device200 is cut is stored into the ROM 620 or the RAM 630.

The drive frequency setting section 612 of the drive control section 210sets the frequency (the drive frequency) of the alternating pulsecurrent, which the lighting circuit 220 outputs, in accordance with themodulation pattern set by the modulation pattern setting section 614. Asdescribed above, the drive frequency is realized by the functions of thedrive frequency setting section 612 and the modulation pattern settingsection 614. Therefore, the drive frequency setting section 612 and themodulation pattern setting section 614 can collectively be called a“drive frequency modulation section” or simply a “frequency modulationsection.” The modulation pattern setting section 614 modifies themodulation pattern to be set, based on the accumulated lighting time ofthe discharge lamp 500 calculated by the lighting time accumulationsection 616. It should be noted that the specific content of themodification of the modulation pattern based on the accumulated lightingtime will be described later.

A3. Drive Frequency Modulation for Discharge Lamp

FIGS. 4A and 4B are explanatory diagrams showing an example of amodulation pattern of the drive frequency fd to be set by the modulationpattern setting section 614. FIG. 4A is a graph showing a time variationof the drive frequency fd. In the modulation pattern shown in FIG. 4A,the drive frequency fd is modulated by varying the drive frequency fd by50 Hz every 4 seconds. Since there are provided 12 periods (retentiveperiods) during which the drive frequency fd is retained at a constantvalue, the period (the modulation period) Tm1 of the modulation of thedrive frequency fd is set to be 48 seconds, and the drive frequency fdof the period (the highest frequency period) Th1 with the highest valueof the drive frequency fd is set to be 350 Hz, and the drive frequencyfd of the period (the lowest frequency period) Tl1 with the lowest valueof the drive frequency fd is set to be 50 Hz.

FIG. 4B shows the time variation of a current (a lamp current) Ipsupplied to the discharge lamp 500 in each of the lowest frequencyperiod Tl1 and the highest frequency period Th1 of the modulationpattern shown in FIG. 4A. In FIG. 4B, the positive direction of the lampcurrent Ip represents the direction of the current flowing from theprimary mirror side electrode 532 toward the secondary mirror sideelectrode 542. Specifically, the primary mirror side electrode 532 actsas an anode in periods Tal, Tah in which the lamp current Ip takes apositive value, while in periods Tcl, Tch in which the lamp current Iptakes a negative value, the primary mirror side electrode 532 acts as acathode. It should be noted that hereinafter the period in which one ofthe electrodes acts as an anode is also referred to as an “anode period”of that electrode, and the period in which one of the electrodes acts asa cathode is also referred to as a “cathode period” of that electrode.

As shown in FIG. 4A, the drive frequency fd (350 Hz) in the highestfrequency period Th1 is set to be 7 times of the drive frequency fd (50Hz) in the lowest frequency period Tl1. Therefore, as shown in FIG. 4B,a switching period Tpl with which the polarity of the lamp current Ip isswitched in the lowest frequency period Tl1 is set to be 7 times as longas the switching period Tph in the highest frequency period Th1.Further, the anode period Tal and the cathode period Tcl of the primarymirror side electrode 532 in the lowest frequency period Tl1 are set tobe 7 times as long as the anode period Tah and the cathode period Tchthereof in the highest frequency period Th1, respectively.

In the first embodiment, a rectangular wave is used as the waveform ofthe lamp current Ip. By thus forming the lamp current Ip as therectangular wave, the absolute value of the lamp current Ip is keptconstant. Therefore, it is prevented that the emitted light intensity ofthe discharge lamp 500 is varied temporally due to the variation of thelamp current Ip. By preventing the temporal variation of the emittedlight intensity, it is possible to prevent occurrence of a phenomenon(scroll noise) that bright and dark fringes appear on the display image.

Further, as shown in FIG. 4B, in the first embodiment, an anode dutyratio of each of the primary mirror side electrode 532 and the secondarymirror side electrode 542 is set to be 50%. Here, the anode duty ratioof the primary mirror side electrode 532 denotes the ratio in length ofthe anode period Tal (Tah) of the primary mirror side electrode 532 withrespect to the switching period Tpl (Tph). Further, the anode duty ratioof the secondary mirror side electrode 542 denotes the ratio in lengthof the anode period of the secondary mirror side electrode 542, namelythe cathode period Tcl (Tch) of the primary mirror side electrode 532,with respect to the switching period Tpl (Tph). It is not necessarilyrequired to set the anode duty ratios of the both electrodes 532, 542 tobe the same. For example, in the case of using the discharge lamp 500having the secondary reflecting mirror 520 as shown in FIG. 2, it isalso possible to set the anode duty ratio of the secondary mirror sideelectrode 542 to be lower than 50%, namely the anode duty ratio of theprimary mirror side electrode 532 to be higher than 50%, inconsideration of the fact that the heat radiation from the secondarymirror side electrode 542 becomes difficult. As described later, sincethe heat generation in an electrode occurs during the anode period ofthe electrode, the value of the heat generated in one of the electrodesincreases as the anode duty ratio of the electrode rises. Therefore,from the viewpoint of the possibility of preventing the excessivetemperature rise of the secondary mirror side electrode 542, the anodeduty ratio of the secondary mirror side electrode 542, which hasdifficulty in heat radiation therefrom, may be set be lower than 50%.

FIGS. 5A, 5B, and 5C are explanatory diagrams showing how the shape ofthe primary mirror side electrode 532 varies when driving the dischargelamp 500 with different drive frequencies fd as shown in FIG. 4A. Asshown in FIG. 5A, the electrodes 532, 542 are respectively provided withprojections 538, 548 toward the opposed electrode. FIG. 5B shows thestate of the primary mirror side electrode 532 in the case in which thedrive frequency fd is low. FIG. 5C shows the state of the primary mirrorside electrode 532 in the case in which the drive frequency fd is high.

FIG. 5A shows the states of the two electrodes 532, 542 in the anodeperiod of the primary mirror side electrode 532. As shown in FIG. 5A, inthe anode period of the primary mirror side electrode 532, electrons areemitted from the secondary mirror side electrode 542 and then collideagainst the primary mirror side electrode 532. In the primary mirrorside electrode 532 acting as the anode, since the kinetic energy of theelectrons having collided is converted into heat energy, the temperaturerises. In contrast, in the secondary mirror side electrode 542 acting asthe cathode, since no collision of the electrons occurs, the temperaturedecreases due to heat conduction and heat radiation. In the similarmanner, in the anode period (i.e., the cathode period of the primarymirror side electrode 532) of the secondary mirror side electrode 542,the temperature of the secondary mirror side electrode 542 rises, whilethe temperature of the primary mirror side electrode 532 decreases.

In the anode period of the primary mirror side electrode 532, since thetemperature of the primary mirror side electrode 532 rises, a meltedportion where the electrode material is melted is caused in theprojection 538 of the primary mirror side electrode 532. Subsequently,when the cathode period of the primary mirror side electrode 532 comes,the temperature of the primary mirror side electrode 532 decreases, andsolidification of the melted portion caused in the tip portion of theprojection 538 begins. By the melted portion thus appearing in each ofthe projections 538, 548 and then being solidified, the projections 538,548 are maintained to have the shape convex toward the opposedelectrode.

FIGS. 5B and 5C show an influence the drive frequency fd exerts on theshapes of the projections. When the drive frequency fd is low,temperature rise occurs in a large area of the projection 538 a of theprimary mirror side electrode 532 in the anode state. Further, when thedrive frequency fd is low, the force applied to the melted portion MRadue to the potential difference from the secondary mirror side electrode542 opposed thereto is also applied to a large area of the meltedsection MRa. Therefore, as shown in FIG. 5B, a flat melted portion MRais formed in the projection 538 of the primary mirror side electrode 532in the anode state. Then, when the primary mirror side electrode 532 isswitched to the cathode state, the melted portion MRa is solidified toform the projection 538 a with a flat shape.

In contrast, when the drive frequency fd is high, the range where thetemperature rise occurs in the projection 538 b of the primary mirrorside electrode 532 in the anode state is reduced, and thus the meltedportion MRb smaller than in the case with the lower drive frequency fdis provided to the projection 538 b. Further, the force applied to themelted portion MRb of the projection 538 b is concentrated to the centerof the melted portion MRb. Therefore, as shown in FIG. 5C, the meltedportion MRb provided to the projection 538 is tapered toward thesecondary mirror side electrode 542 opposed thereto, and therefore, theshape of the projection 538 b obtained by solidifying the melted portionMRb in the cathode period also becomes tapered.

As described above, in the case in which the drive frequency fd is low,since the projection 538 a is sufficiently melted, the projection 538 abecomes large. In contrast, in the case in which the drive frequency fdis high, extension of the projection 538 b toward the opposed electrodeis promoted. Therefore, by modulating the drive frequency fd, theprojection 538 a becomes large in the low frequency driving mode withthe lower drive frequency fd, and the projection 538 b extends in thehigh frequency driving mode with the higher drive frequency fd. Thus,the distance between the electrodes 532, 542 can be prevented fromincreasing, and the voltage between the electrodes 532, 542 can beprevented from rising when supplying the discharge lamp 500 with thealternating pulse current of constant power.

Further, as shown in FIG. 4A, the drive frequency fd varies stepwisefrom 50 Hz in the lowest frequency period Tl1 to 350 Hz in the highestfrequency period Th1. Therefore, since the projection 538 a, which hasonce become larger, sequentially changes to have the tapered shape, theprojection with a shape such as a conical shape suitable for stabilizingthe position at which the arc occurs can be formed. It should be notedthat in the case in which the frequency is sequentially varied along themodulation pattern shown in FIG. 4A, since the phenomenon in the case inwhich the drive frequency fd is high and the phenomenon in the case ofthe low drive frequency are repeated continuously, it looks to the eyethat the projections 538, 548 on the tips of the electrodes 532, 542 arekept to have shapes suitable for stabilizing the position at which thearc occurs, such as a conical shape, and it is difficult to observe theactual state in which the projection 538 a becomes large in the lowfrequency driving mode and the projection 538 b extends in the highfrequency driving mode.

It should be noted that since the shape of the projection 538 b isbecoming tapered in the high frequency driving mode, there is apossibility that miniaturization of the projection 538 b is advanced toform a minute projection. If the minute projection is formed, theremight be caused a flicker (arc-jump) in which the position of occurrenceof the arc migrates due to deformation of the minute projection orpluralization of the minute projection. However, in the firstembodiment, the minute projection disappears in the low frequencydriving mode by modulating the drive frequency fd. Therefore, theflicker caused by the formation of the minute projection may beeliminated. On the other hand, in the low frequency driving mode, sincethe shape of the projection 538 a becomes planarized, the possibility ofgenerating the flicker rises over time. FIGS. 6A and 6B are explanatorydiagrams showing an influence exerted during a passage of time on ageneration state of the arc in the low frequency driving mode.

FIG. 6A shows the generation state of the arc AR at the time point(beginning of the low frequency driving mode) when the drive frequencyfd is switched from the high state to the low state. FIG. 6B shows thegeneration state of the arc AR at the time point (during the lowfrequency driving mode) when a predetermined time has elapsed from thebeginning of the low frequency driving mode.

As described above, in the case when the drive frequency fd is high, thearea of the projection 538 b in the anode state the temperature of whichrises becomes small, and the projection 538 b with a tapered shape isformed. Therefore, as shown in FIG. 6A, at the beginning of the lowfrequency driving mode, the high temperature area HRa (a hot spot) isformed at the tip of the projection, and the projection 538 a extendstowards the side of the secondary mirror side electrode 542 (FIG. 6A)opposed thereto. The arc AR occurs from a position at which the electrone⁻ is emitted in the cathode period of the electrode 532. The higher thetemperature is, and the stronger the electric field is, the more easilythe electron e⁻ is emitted from the cathode. Therefore, as shown in FIG.6A, the electron e⁻ is emitted from the tip of the projection 538 a, andthe arc AR occurs from the tip of the projection 538 a.

In contrast, in the low frequency driving mode, the area the temperatureof which rises extends, as described above. Therefore, as shown in FIG.6E, the hot spot HRc extends during the low frequency driving mode.Further, since the projection 538 c is flattened due to the lowfrequency driving, the tip of the projection 538 c is flattened. Asdescribed above, since the hot spot HRc extends, and the tip of theprojection 538 c is flattened, the positions from which the electron e⁻is easily emitted become distributed in a large area. Therefore, the arcAR occurs from a random position of the projection 538 c thus planarizedduring the low frequency driving mode, and the possibility of occurrenceof the flicker rises.

Further, in the discharge lamp 500, the electrodes 532, 542 areconsumed, and the tips thereof are planarized as the accumulatedlighting time becomes longer. Therefore, if the period of the lowfrequency driving is set to be long in the condition in which theaccumulated lighting time is long, the possibility of occurrence of theflicker rises. Therefore, in the first embodiment, in order forpreventing the flicker from occurring, the modulation pattern of thedrive frequency fd is modified in accordance with the accumulatedlighting time.

A4. Modification of Modulation Pattern

FIG. 7 is a diagram showing how the modulation pattern is modified inthe first embodiment. It should be noted that the modification of themodulation pattern is realized as a function of the modulation patternsetting section 614. The modulation pattern setting section 614 (FIG. 3)can be arranged to select the modulation pattern to be set from aplurality of modulation patterns stored previously in the ROM 620, or tocopy the modulation pattern, which is stored in the ROM 620, on the RAM630, and then modify the modulation pattern thus copied.

The graph shown in FIG. 7 shows a time variation in the drive frequencyfd around the time point at which the accumulated lighting time exceedsa predetermined amount of time (500 hours in the example shown in FIG.7). As shown in FIG. 7, the modulation pattern of the drive frequency fdis modified before and after the time point at which the accumulatedlighting time exceeds 500 hours. It should be noted that although in theexample shown in FIG. 7, the modification of the modulation pattern isexecuted when the drive frequency fd is switched from 200 Hz to 250 Hz,it is possible to execute the modification of the modulation pattern atany point of time. It should be noted that the modulation pattern may bemodified when the drive frequency fd is switched. As described above, inthe first embodiment, since the modulation pattern is modified when theaccumulated lighting time reaches the predetermined time, the time pointwith the accumulated lighting time equal to or shorter than thepredetermined time is also described as “before modification,” and thetime point with the accumulated lighting time exceeding thepredetermined time is also described as “after modification.”

In FIG. 7, the modulation pattern before modification is the same asshown in FIG. 4A. Further, when the accumulated lighting time exceeds500 hours, the length (step time) of the retentive period during whichthe drive frequency fd is retained constant is uniformly modified to be2 seconds, which is a half as long as before modification, independentlyof the drive frequency fd. In the modulation pattern after modification,the drive frequencies fd (50 Hz and 350 Hz) in the lowest frequencyperiod Tl2 and the highest frequency period Th2, and the variation step(50 Hz) of the drive frequency fd are the same as those of themodulation pattern before modulation, respectively. Therefore, themodulation period Tm2 (24 seconds) in the modulation pattern aftermodification is arranged to be a half of the modulation period Tm1 (48seconds) in the modulation pattern before modification. It should benoted that hereinafter the modulation pattern with a shortened retentiveperiod like the modulation pattern after modification is also referredto as a “shortened pattern,” and the modulation pattern beforemodification with a retentive period not shortened is also referred toas a “stationary pattern.”

As described above, in the first embodiment, in the case in which theaccumulated lighting time exceeds the predetermined time, it is arrangedthat the length of each of the retentive periods becomes shorter thanthe length of the retentive period before modification. Therefore, theperiod of time during which the low frequency driving is performed canbe shortened, thus the flicker can be prevented from occurring. Itshould be noted that although in the first embodiment, the length of theretentive period after modification is set to be a half of the length ofthe retentive period before modification, in general, it is sufficientthat the length becomes shorter than before modification. The extent towhich the length of the retentive period after modification is shortenedfrom that before modification is appropriately determined based on anexperiment and so on.

Although in the first embodiment, the modification of the modulationpattern of the drive frequency fd is performed in accordance with theaccumulated lighting time, in general, it is also possible to arrangethat the modulation pattern is modified in accordance with thedeterioration state of the discharge lamp 500 such as the state in whichthe electrodes 532, 542 are consumed. The deterioration state of thedischarge lamp 500 can be detected based on the lamp voltage, reductionin the light intensity due to the evaporation of the electrode materialon the inner wall of the discharge space 512 (FIG. 2), and so on,besides the accumulated lighting time. The lamp voltage can be detectedby appropriately configuring the lighting circuit 220. Further, thereduction in the light intensity can be detected by providing a lightreceiving element in the light path of the light emitted from thedischarge lamp 500.

B. Second Embodiment

FIG. 8 is a diagram showing how the modulation pattern is modified inthe second embodiment. The second embodiment is different from the firstembodiment in the modulation patterns before and after modification,namely in the stationary pattern and the shortened pattern. The otherpoints are the same as in the first embodiment.

As shown in FIG. 8, in the stationary pattern of the second embodiment,the retentive period during which the drive frequency fd is retainedconstant is set to be 6 seconds independently of the drive frequency fd.Further, in the stationary pattern of the second embodiment, the drivefrequency fd in the lowest frequency period Tl3 is set to be 100 Hz, andthe drive frequency fd in the highest frequency period Th3 is set to be300 Hz. The variation step of the drive frequency fd is set to be 50 Hzsimilarly to the first embodiment. Therefore, the modulation period Tm3in the stationary pattern in the second embodiment is arranged to be 48seconds.

On the other hand, in the shortened pattern of the second embodiment,the retentive period is shortened by 1 second from 6 seconds in thehighest frequency period Th4 to 2 seconds in the lowest frequency periodTl4, as the drive frequency fd becomes lower. The drive frequencies fd(100 Hz and 300 Hz) in the lowest frequency period Tl4 and the highestfrequency period Th4, and the variation step (50 Hz) of the drivefrequency fd are the same as those of the stationary pattern,respectively. Therefore, the modulation period Tm4 in the shortenedpattern in the second embodiment is arranged to be 32 seconds. It shouldbe noted that although in the shortened pattern in the secondembodiment, the retentive period is shortened by 1 second every time thedrive frequency fd decreases by 50 Hz, it is not necessarily requiredthat the amount of shortening of the retentive period is constant, butcan appropriately be set for every value of the drive frequency fd.

As described above, in the second embodiment, when the accumulatedlighting time exceeds the predetermined time, the modulation pattern ofthe drive frequency is set to be the shortened pattern with theretentive period the length of which is shortened as the drive frequencyfd becomes lower. Therefore, the period of time during which the lowfrequency driving is performed can be shortened, thus the flicker can beprevented from occurring. Further, according to the second embodiment,sufficiently long period of time during which the high frequency drivingis performed can be obtained. Therefore, since the extension of theprojection 538 b in the high frequency driving mode can sufficiently beperformed as shown in FIG. 5C, the distance between the electrodes 532,542 can be prevented from increasing to raise the lamp voltage.

It should be noted that although in the second embodiment, the retentiveperiod is shortened as the drive frequency fd becomes lower, in general,it is sufficient that the length of the retentive period at a certaindrive frequency is shorter than the length of the retentive period at adrive frequency higher than the certain drive frequency. Also in such aconfiguration, it is possible to shorten the period of time during whichthe low frequency driving is performed, and at the same time, toincrease the period of time during which the high frequency driving isperformed. Further, although in the second embodiment, the length of thehighest frequency period Th3 of the stationary pattern and the length ofthe highest frequency period Th4 of the shortened pattern are arrangedto be the same, it is not necessarily required that the lengths of theperiods Th3, Th4 are equal to each other. If the retentive period at afrequency lower than a predetermined frequency in the shortened patternis shorter than the retentive period at the frequency lower than thepredetermined frequency in the stationary pattern, the highest frequencyperiod Th4 in the shortened pattern can be longer than the highestfrequency period Th3 in the stationary pattern, or the highest frequencyperiod Th3 in the stationary pattern can be longer than the highestfrequency period Th4 in the shortened pattern.

C. Third Embodiment

FIG. 9 is a block diagram showing a configuration of a discharge lampdriving device 200 a in a third embodiment. The discharge lamp drivingdevice 200 a of the third embodiment is different from the dischargelamp driving device 200 (FIG. 3) of the first embodiment in the pointthat a flicker sensor 700 disposed adjacently to the discharge lamp 500is provided, the point that the drive control section 210 a is providedwith a sensor interface 670 to which an output signal from the flickersensor 700 is supplied, and the point that the CPU 610 a realizes afunction as a flicker detection processing section 618 instead of thelighting time accumulation section 616. Further, the modulation patternsetting section 614 a is different from the modulation pattern settingsection 614 of the first embodiment in the point that the modulationpattern is modified in accordance with the state of occurrence of theflicker detected by the flicker detection processing section 618. Theother points are substantially the same as in the first embodiment.

The flicker sensor 700 detects the variation in the position of the arcAR (FIG. 2) occurring in the discharge lamp 500. The flicker sensor 700in the third embodiment can be composed of a light receiving elementsuch as a photodiode or a phototransistor, and a slit or a pinhole forvarying the light intensity of the light entering the light receivingelement in accordance with the variation in the position at which thearc occurs. It should be noted that the configuration of the flickersensor 700 can arbitrarily be modified providing that the variation inthe position at which the arc occurs can be detected. For example, asthe flicker sensor 700 for detecting the variation in the position ofthe arc, a line sensor or an area sensor composed of charge-coupleddevices (CCD) or the like can also be used.

The flicker detection processing section 618 analyzes the output signalfrom the flicker sensor 700 acquired via the sensor interface 670 todetect the state of occurrence of the flicker in the discharge lamp 500.The state of occurrence of the flicker can be detected based on theamount of variation in the light intensity of the light entering thelight receiving element provided to the flicker sensor 700, for example.In this case, if the amount of variation exceeds a predeterminedreference amount, it is determined that the flicker occurs. It should benoted that the detection of the flicker in the flicker detectionprocessing section 618 is appropriately modified in accordance with theconfiguration of the flicker sensor 700.

FIG. 10 is a flowchart showing a flow of a process of the modulationpattern setting section 614 a of the third embodiment modifying themodulation pattern. The process of modifying the modulation pattern isrepeatedly executed during the period in which the discharge lamp 500 isin the lighting state.

In the step S110, the modulation pattern setting section 614 a obtainsthe state of occurrence of the flicker. Specifically, the modulationpattern setting section 614 a obtains the state of occurrence of theflicker from the flicker detection processing section 618.

In the step S120, the modulation pattern setting section 614 adetermines whether or not the flicker occurs in the retentive periodwith the drive frequency equal to or lower than a predeterminedreference frequency (e.g., 200 Hz). If it is determined that the flickeroccurs in the retentive period with the drive frequency equal to orlower than the reference frequency, the process proceeds to the stepS130. On the other hand, if it is determined that the flicker does notoccur in the retentive period with the drive frequency equal to or lowerthan the reference frequency, the process returns to the step S110, andthe two steps S110, S120 are executed repeatedly.

In the step S130, the modulation pattern setting section 614 a modifiesthe modulation pattern, thereby shortening the retentive period with thedrive frequency equal to or lower than the frequency at which theflicker occurs. The modification of the modulation pattern is performedwhen the drive frequency fd is switched from 200 Hz to 250 Hz similarlyto the case of the first embodiment. It should be noted that themodification of the modulation pattern can be performed at an arbitrarypoint of time.

In the step S140, the modulation pattern setting section 614 adetermines whether or not predetermined time has elapsed after modifyingthe modulation pattern. If it is determined that the predetermined timehas elapsed, the process proceeds to the step S150. On the other hand,if it is determined that the predetermined time has not yet elapsed, thestep S140 is executed repeatedly until the predetermined time elapses.It should be noted that the predetermined time is set to be the time(e.g., 10 through 20 minutes) necessary for the shape of the projectionto change to the extent that the occurrence of the flicker in the lowfrequency driving mode can be prevented.

In the step S150, the modulation pattern setting section 614 a obtainsthe state of occurrence of the flicker similarly to the step S110. Then,in the step S160, the modulation pattern setting section 614 adetermines whether or not the flicker, which has been determined tooccur in the step S120, is eliminated. If it is determined that theflicker has been eliminated, the process proceeds to the step S170. Onthe other hand, if it is judged that the flicker has not yet beeneliminated, the process returns to the step S140, and the steps S140through S160 are executed until the flicker is eliminated. It should benoted that when making the determination on whether or not the flickeris eliminated, one may temporarily restore the length of the shortenedretentive to the length thereof before the shortening in order fordetermining the elimination of the flicker more reliably.

In the step S170, the modulation pattern setting section 614 a restores(hereinafter also referred to as “restoration”) the length of theretentive period, which was shortened in the step S130, to its originallength. The restoration of the retentive period can be performed by, forexample, changing the modulation pattern to be set into the drivefrequency setting section 612 to the modulation pattern beforeshortening stored in the ROM 620 or the RAM 630. After the restorationof the retentive period in the step S170, the process returns to thestep S110, and the steps S110, S120 are executed repeatedly until theflicker occurs in the retentive period with the drive frequency equal toor lower than the reference frequency.

FIG. 11 is an explanatory diagram showing how the modulation pattern ismodified by the modification process for the modulation pattern shown inFIG. 10. The modulation pattern before shortening the retentive period,namely the stationary pattern, shown in FIG. 11 is the same as in thesecond embodiment.

In the example shown in FIG. 11, no flicker occurs prior to the timepoint t1. Then, in the time period between the time points t1 and t2,the flicker occurs. Therefore, it is determined that the flicker occursin the retentive period started from the time point t1 with the drivefrequency fd of 150 Hz, namely at the drive frequency lower than thereference frequency (200 Hz) (step S120). Subsequently, the modulationpattern is modified to be the shortened pattern at the time point t3 atwhich the drive frequency fd is switched from 200 Hz to 250 Hz (stepS130).

In the example shown in FIG. 11, since the flicker occurs at the drivefrequency fd of 150 Hz, in the shortened pattern the retentive periodswith the drive frequencies fd of 150 Hz and 100 Hz are shortened to be 2seconds, namely a third as long as in the stationary pattern. Therefore,the modulation period Tm5 in the shortened pattern becomes 36 seconds.It should be noted that the extent of the shortening of the retentiveperiod can appropriately be modified.

As described above, in the third embodiment, by shortening the retentiveperiods with the drive frequencies equal to or lower than the frequencyat which the flicker occurs, the period of time during which the lowfrequency drive is performed is shortened, thus it becomes possible toprevent the flicker from occurring. Further, since according also to thethird embodiment, it is possible to provide sufficiently long period oftime during which the high frequency drive is executed, rise in the lampvoltage can be prevented.

Although in the third embodiment, all of the retentive periods with thedrive frequencies fd equal to or lower than the frequency at which theflicker occurs are shortened, it is also possible to arrange that onlythe retentive periods with the drive frequency equal to the frequency atwhich the flicker occurs are shortened. Further, it is also possible toarrange that if the flicker occurs at a frequency equal to or lower thanthe reference frequency, all of the retentive periods with the drivefrequencies equal to or lower than the reference frequency areshortened. In general, when the flicker occurs, one may shorten at leastone retentive period with the drive frequency equal to or lower than thereference frequency, it is also possible to shorten all of the retentiveperiods similarly to the shortened pattern of the first embodiment, orit is also possible to arrange that the retentive period is shortened asthe drive frequency thereof becomes lower similarly to the shortenedpattern in the second embodiment.

Further, in the third embodiment, although whether or not the flicker iseliminated is determined when the predetermined time has elapsed afterthe modulation pattern is modified to be the shortened pattern (stepsS140 through S160), it is also possible to arrange that whether or notthe flicker is eliminated is determined without waiting the elapse ofthe predetermined time. By determining the elimination of the flickerafter the predetermined time has elapsed in the point that the taperedprojection 538 b (FIG. 5C) is formed until the predetermined timeelapses, it is possible to prevent the reoccurrence of the flicker whenthe retentive period is restored.

Although in the third embodiment, the length of the retentive periodonce shortened is restored when the flicker is eliminated, it is alsopossible to arrange that the restoration of the length of the retentiveperiod is not executed. In this case, it is also possible to arrangethat the shortened retentive period is kept until, for example, thedischarge lamp 500 is put off, or the discharge lamp driving device 200a is powered off, and then the length of the retentive period isrestored at the beginning of lighting of the discharge lamp 500, or whenthe discharge lamp driving device 200 a is powered on. Further, it isalso possible to arrange that the restoration of the length of theretentive period is not performed at all. One may restore the shortenedlength of the retentive period when the flicker is eliminated to thepoint that the minute projection, which can be formed in the highfrequency driving mode, can be eliminated by restoring the length of theretentive period. As such, the occurrence of the flicker due to theformation of the minute projections can be prevented.

D. Fourth Embodiment

FIG. 12 is a diagram showing how the modulation pattern is modified in afourth embodiment. The fourth embodiment is different from the firstembodiment in the modulation pattern around the modification. The otherpoints are the same as in the first embodiment.

As shown in FIG. 12, in the stationary pattern of the fourth embodiment,the retentive period during which the drive frequency fd is retainedconstant is set to be 4 seconds independently of the drive frequency fd.Further, in this stationary pattern, the drive frequency fd in thelowest frequency period Tl1 is set to be 50 Hz, and the drive frequencyfd in the highest frequency period Th1 is set to be 350 Hz. Thevariation step of the drive frequency fd is set to be 50 Hz similar tothe first embodiment. Therefore, the modulation period Tm1 in thestationary pattern in the fourth embodiment is arranged to be 48seconds.

In the fourth embodiment, when the accumulated lighting time exceeds apredetermined period of time (500 hours in the example shown in FIG.12), the modulation pattern of the drive frequency is set to be anelongated pattern with the highest frequency period Th1 longer than inthe case of the stationary pattern. In other words, the highestfrequency period Th3 is changed from 4 seconds corresponding to thestationary pattern to 10 seconds corresponding to the elongated pattern.In this condition, the retentive periods corresponding respectively tothe drive frequencies fd of 300 Hz and 250 Hz are changed to 2.5 secondsso that the modulation period of the drive frequency fd becomes the samebetween the case of the stationary pattern and the case of the elongatedpattern of the drive frequency.

In general, if the electrode is deteriorated and the fusibility thereofis degraded, the projection may begin to have a shape with a flat tiphaving a large thermal capacity when the drive frequency fd is low, andunevenness may be formed on the surface of the projection. As such, thepossibility of occurrence of the flicker rises. In contrast, in thefourth embodiment having the configuration described above, since thereis adopted the configuration in which the highest frequency period Th1becomes longer when a predetermined condition is satisfied (for example,in the fourth embodiment, when the accumulated lighting time exceeds thepredetermined time) compared to the case in which the predeterminedcondition has not been satisfied yet, the minute projection can beformed to reduce the thermal capacity in the highest frequency periodTh1. Thus, the fusibility of the surface of the projection may bemaintained. Therefore, it becomes possible to prevent the flicker fromoccurring.

E. Fifth Embodiment

FIG. 13 is a diagram showing how the modulation pattern is modified in afifth embodiment. The fifth embodiment is different from the secondembodiment in the modulation pattern around the modification. The otherpoints are the same as in the second embodiment. In the fourthembodiment, the modulation pattern (the elongated pattern) after themodification is configured to have the highest frequency period Th3longer than in the case of the stationary pattern. In contrast, in thefifth embodiment, the elongated pattern is configured so that all of theretentive periods with the frequency higher than the predeterminedreference frequency are longer than in the case of the stationarypattern.

In the fifth embodiment, as shown in FIG. 13, the retentive periods withthe drive frequencies of 250 Hz and 300 Hz, respectively, exceeding thepredetermined reference frequency (e.g., 200 Hz) are changed from 6seconds corresponding to the stationary pattern to 10 secondscorresponding to the elongated pattern. In the case in which the drivefrequency is equal to or lower than the reference frequency, theretentive periods are 6 seconds without the change in both of thestationary pattern and the elongated pattern.

In the fifth embodiment having the configuration described above,similar to the fourth embodiment, the unevenness on the surface of theprojection generated when the drive frequency fd is low can be reduced.Therefore, it becomes possible to prevent the flicker from occurring.

F. Modified Examples

It should be noted that the disclosure is not limited to the embodimentsor the specific examples described above, but can be put into practicein various forms within the scope or the spirit of the disclosure. Byway of example, the following modifications are also possible.

F1. Modified Example 1

The modulation pattern shown in each of the embodiments described aboveis nothing more than an example, and the modulation range (i.e., thedrive frequencies in the highest frequency periods Th1 through Th5 andthe lowest frequency periods Tl1 through Tl5) of the drive frequency,the number of retentive periods during which the drive frequency isretained constant, the length of the retentive period, the variationamount of the drive frequency, and so on can appropriately be modifiedin accordance with the characteristic of the discharge lamp 500 and soon. In general, one may perform the modulation of the drive frequency byproviding a plurality of periods having the drive frequencies differentfrom each other in the modulation period. For example, it is alsopossible that the lengths of the retentive periods can be differentbetween the drive frequencies, and further, it is also possible toarrange that the drive frequency is not switched stepwise. Also in sucha configuration, occurrence of the flicker in the low frequency drivingmode can be prevented by shortening the retentive period with the drivefrequency equal to or lower than the predetermined frequency. Further,it is also possible to adopt the configuration in which only the lowestfrequency period is shortened in the case in which the predeterminedcondition is satisfied, compared to the case in which the predeterminedcondition has not been satisfied yet.

F2. Modified Example 2

Although the configuration of shortening at least one period having thefrequency equal to or lower than the predetermined reference frequencyis adopted in the embodiments 1 through 3, and the configuration ofelongating at least one period having the frequency exceeding thepredetermined reference frequency is adopted in the embodiment 4, the“predetermined reference frequency” is not limited to 200 Hz, but canarbitrarily be modified in accordance with the characteristic of thedischarge lamp 500 providing the flicker is apt to occur at thefrequency. Further, the “predetermined reference frequency” in the firstthrough third embodiments shortening the period and the “predeterminedreference frequency” in the fourth and fifth embodiments elongating theperiod are not necessarily the same, but can take values different fromeach other.

F3. Modified Example 3

Although in each of the embodiments described above, the modulationpattern is switched from the stationary pattern to the shortened patternbased on the deterioration state of the discharge lamp 500 and theactual occurrence of the flicker, it is also possible to switch themodulation pattern based on other conditions. For example, in the casein which the discharge lamp driving device 200 is configured to be ableto drive the discharge lamp 500 with power lower than the rated power,it is also possible to arrange that the stationary pattern is used whendriving the discharge lamp 500 with the rated power, while the shortenedpattern is used when driving the discharge lamp 500 with the power (lowpower) lower than the rated power. It should be noted that the powerused as the basis of the switching of the modulation pattern is notnecessarily limited to the rated power. In general, it is also possibleto arrange that the stationary pattern is used in the case in which thedrive power of the discharge lamp is equal to or greater than apredetermined reference power, and the shortened pattern is used in thecase in which the drive power of the discharge lamp is smaller than thepredetermined reference power.

FIGS. 14A and 14B are explanatory diagrams showing how the arc AR occurswhen the low frequency drive is continued in a low power driving mode.FIG. 14A shows the state of occurrence of the arc AR at the beginning ofthe low frequency driving mode, and FIG. 14B shows the state ofoccurrence of the arc AR during the low frequency driving mode.

As described above, in the case in which the drive frequency fd is high,the area in the projection 538 d in the anode state the temperature ofwhich rises becomes small. Therefore, also in the low power drivingmode, the hot spot HRd with sufficiently high temperature is formed atthe tip of the projection at the beginning of the low frequency drivingmode as shown in FIG. 14A. As described above, since the hot spot HRd isprovided to the projection 538 d, the arc AR occurs from the position ofthe hot spot HRd, namely the tip of the projection 538 d.

In contrast, when continuing the low frequency driving in the low powerdriving mode in which the temperature of the entire electrode 532 drops,since the area the temperature of which rises extends due to the lowfrequency driving, the temperature of the tip of the projection 538 ddoes not rise sufficiently. Therefore, during the low frequency drivingmode, the temperature of the tip of the projection 538 d does notsufficiently rise, and the hot spot HRd formed at the beginning of thelow frequency driving mode disappears as shown in FIG. 14B. If the hotspot HRd with the sufficiently high temperature disappears as describedabove, it becomes that the position from which the electron e⁻ is apt tobe emitted is not fixed, and the arc AR occurs from a random position onthe projection 538 d, and therefore, the possibility of occurrence ofthe flicker becomes higher.

By thus using the stationary pattern as the modulation pattern in therated power driving mode and using the shortened pattern in the lowpower driving mode, it becomes possible to prevent the flicker fromoccurring in the low power driving mode.

It should be noted that it is also possible to perform switching of themodulation pattern based on a plurality of conditions instead of asingle condition. For example, it is also possible to arrange that inthe rated power driving mode, switching of the modulation pattern isperformed based on the deterioration state of the discharge lamp 500 andthe occurrence of the flicker, and in the low power driving mode, theshortened pattern is used irrespectively of the deterioration state ofthe discharge lamp 500 or the occurrence of the flicker. Further, thebasis used when switching the modulation pattern based on thedeterioration state of the discharge lamp 500 can be made differentbetween the rated power driving mode and the low power driving mode.

F4. Modified Example 4

FIGS. 15A and 15B are explanatory diagrams showing a modified example ofthe electrode provided to the discharge lamp in comparison with theembodiments described above. FIG. 15A shows a shape of the electrode 532(542) in the embodiments described above, and FIG. 15B shows a shape ofthe electrode in the modified example 4. As shown in FIG. 15A, the tipof the electrode 532 (542) in the embodiments described above forms aspherical surface. FIGS. 5A through 5C, and 6A and 6B explained abovealso show that the tip has a spherical shape. As described above, itbecomes possible to enhance the effect of restoring the modification inthe deterioration by making the tip of the electrode have a sphericalshape.

In contrast, in FIG. 15B, the tip of the electrode 1532 has a conicalshape. According to this configuration, the flicker can also beprevented from occurring similarly to each of the embodiments. In otherwords, the tip of the electrode 1532 is not required to have a sphericalshape, but can be made to have various shapes such as a conical shape.

F5. Modified Example 5

Although in each of the embodiments described above the liquid crystallight valves 330R, 330G, 330B are used as the light modulation sectionsin the projector 1000 (FIG. 1), it is also possible to use otherarbitrary modulation sections such as digital micromirror devices (DMD,a trademark of Texas Instruments) as the light modulation sections.Further, the disclosure can also be applied to various types of imagedisplay devices, exposure devices, illumination devices, and so onincluding the liquid crystal display devices as long as the devices usethe discharge lamp as the light source. Therefore, it is manifestlyintended that embodiments in accordance with the present disclosure belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A driving device for a discharge lamp,comprising: an alternating current supply section configured to supplytwo electrodes of the discharge lamp with an alternating current, thealternating current comprising a plurality of modulation periods; and afrequency modulation section configured to modulate a frequency of thealternating current in accordance with a predefined modulation pattern,the predefined modulation pattern comprising a plurality of retentiveperiods within each of the modulation periods, each retentive periodhaving a constant frequency that is different from a frequency oftemporally adjacent retentive periods, wherein the frequency modulationsection makes a length of at least one of the retentive periods in themodulation period shorter in response to at least two conditions beingmet: (a) a predetermined condition occurring, and (b) the frequency ofthe at least one of the retentive periods is equal to or less than apredetermined reference frequency, and the frequency modulation sectionis configured to modulate the frequency of the alternating current inaccordance with the predefined modulation pattern when the at least twoconditions are not met.
 2. The driving device according to claim 1,wherein the each of the modulation periods includes a first retentiveperiod having a first frequency and a second retentive period having asecond frequency, the second frequency being higher than the firstfrequency, and wherein the shortening of the retentive periods includesshortening a length of the first retentive period to be shorter than alength of the second retentive period.
 3. The driving device accordingto claim 1, wherein the shortening of the retentive periods includesshortening a length of the retentive period in accordance with thefrequency of the retentive period such that the lower the frequency theshorter the length of the retentive period.
 4. The driving deviceaccording to claim 1, wherein the frequency modulation section shortenseach of the retentive periods in the modulation period in response tothe occurrence of the predetermined condition.
 5. The driving deviceaccording to claim 1, further comprising a flicker detection processingsection configured to detect an occurrence of a flicker in the dischargelamp as a criterion of the predetermined condition, wherein thefrequency modulation section performs the shortening when the flicker isdetected in the retentive periods with the frequency equal to or lessthan the reference frequency.
 6. The driving device according to claim5, wherein after a predetermined time has elapsed since the shorteningof the retentive periods, the frequency modulation section determineswhether the flicker still occurs at the frequency at which the flickerhad been detected, and if not, the frequency modulation section restoresthe length of the shortened retentive period to a length prior to theshortening.
 7. The driving device according to claim 1, wherein thepredetermined condition is determined based on a deterioration state ofthe discharge lamp, and the frequency modulation section performs theshortening in response to a determination that a deterioration of thedischarge lamp is in progress.
 8. The driving device according to claim7, further comprising: a lighting time accumulation section configuredto calculate an accumulated lighting time from a beginning of use of thedischarge lamp as a parameter representing the deterioration state ofthe discharge lamp, and wherein the frequency modulation sectionperforms the shortening in response to the accumulated lighting timeexceeding a predetermined upper time limit.
 9. The driving deviceaccording to claim 1, wherein the alternating current supply section isconfigured to modify a power supplied to the discharge lamp, and thefrequency modulation section performs the shortening in response to thepower supplied to the discharge lamp being lower than a predeterminedreference power.
 10. An image display apparatus comprising: a dischargelamp as a light source configured to display an image; an alternatingcurrent supply section configured to supply two electrodes of thedischarge lamp with an alternating current to light the discharge lamp,the alternating current comprising a plurality of modulation periods;and a frequency modulation section configured to modulate a frequency ofthe alternating current in accordance with a predefined modulationpattern, the predefined modulation pattern comprising a plurality ofretentive periods within each of the modulation periods, each retentiveperiod having a constant frequency that is different from a frequency oftemporally adjacent retentive periods, wherein the frequency modulationsection makes a length of at least one of the retentive periods in themodulation period shorter in response to at least two conditions beingmet: (a) a predetermined condition occurring, and (b) the frequency ofthe at least one of the retentive periods is equal to or less than apredetermined reference frequency, and the frequency modulation sectionis configured to modulate the frequency of the alternating current inaccordance with the predefined modulation pattern when the at least twoconditions are not met.
 11. The image display apparatus according toclaim 10, wherein the each of the modulation periods includes a firstretentive period having a first frequency and a second retentive periodhaving a second frequency, the second frequency being higher than thefirst frequency, and wherein the shortening of the retentive periodsincludes shortening a length of the first retentive period to be shorterthan a length of the second retentive period.
 12. The image displayapparatus according to claim 10, wherein the predetermined condition isbased on a deterioration state of the discharge lamp, and the frequencymodulation section performs the shortening in response to adetermination that a deterioration of the discharge lamp is in progress.13. The image display apparatus according to claim 10, wherein thealternating current supply section is configured to modify a powersupplied to the discharge lamp, and the frequency modulation sectionperforms the shortening in response to the power supplied to thedischarge lamp being lower than a predetermined reference power.
 14. Adriving method for a discharge lamp, comprising: supplying analternating current between two electrodes of the discharge lamp, thealternating current comprising a plurality of modulation periods;modulating a frequency of the alternating current in accordance with apredefined modulation pattern, the predefined modulation patterncomprising a plurality of retentive periods within each of themodulation periods, each retentive period having a constant frequencythat is different from a frequency of temporally adjacent retentiveperiods; and shortening a length of at least one of the retentiveperiods in the modulation period in response to at least two conditionsbeing met: (a) a predetermined condition occurring, and (b) thefrequency of the at least one of the retentive periods is being equal toor less than a predetermined reference frequency, wherein the modulatingof the frequency of the alternating current is performed in accordancewith the predefined modulation pattern when the at least two conditionsare not met.